Seismic sources, both vibratory and impulsive, have long been used in the seismic data acquisition industry to generate the acoustic signals needed in geophysical exploration. The conventional use of seismic sources involves several well-understood steps. In the case of vibratory sources, first, one or more vibrators are located at a source point on the surface of the earth. Second, the vibrators are activated for a period of time, typically ranging from four to sixteen seconds, and generate a replicate of the pilot signal. The pilot signal is typically a sweep signal that varies in frequency during the period of time in which the vibrators are activated. Third, seismic receivers are used to receive and record response data for a period of time equal to the sweep time plus a listen time. The period of time over which data is recorded includes at a minimum the time necessary for the seismic signals to travel to and reflect off of the target reflectors of interest, and for the reflected signals to return to the receivers. The recorded signals contain weighted, summed, time delayed, phase rotated and filtered versions of the pilot signal, signals harmonically related to the pilot signal, instrumentation noise and ambient noise. Fourth, seismograms are generated by compressing the longer duration pilot signal to a center weighted wavelet. The most typical means of compressing the recorded data is via cross-correlation with either the pilot signal or a reference sweep. Fifth, the sweep and correlation steps are repeated several times, typically four to eight, and the compressed data are added together in a process referred to as stacking. Finally, the vibrators are moved to a new source point and the entire process is repeated using identically the same acquisition parameters or possibly an alternate predetermined parameter set.
Several problems are known to exist with conventional vibrator technology. First, the correlation process is known to result in correlation side lobes, which can influence the accuracy of the final processed data. Second, vibrator harmonic distortion results in noise, known as harmonic ghosts, after correlation with the pilot. A partial solution to this problem is the use of upsweeping pilot signals, in which the sweep starts at low frequencies and increases to high frequencies. This approach places the correlation ghosts before the main correlation peak where they will not interfere with later, and hence weaker, reflections. In addition, to minimize noise from harmonics, multiple sweeps are performed with incremental phase rotation of the sweeps so that after correlation and stack, the amplitude of the harmonics are reduced. For example, a typical practice to suppress harmonics through the fourth order harmonic would be to use four sweeps with a phase rotation of 360 degrees divided by four, i.e., 0, 90, 180, and 270 degrees. The data are stacked after correlation, with the amplitude of the harmonics accordingly reduced, although not eliminated. The harmonic energy generated by marine and land vibratory sources has traditionally been considered a problem to be eliminated. This is especially true for marine sources when considering the effects of higher frequency signals on ocean mammals. Generally the effects of the harmonic energy is simply ignored or suppressed via adaptive feed forward techniques. Third, in order to accurately process the recorded data, both the sweep time and a listen time must be included in the recording time of the seismic receivers for each sweep. The listen time is important to ensure that the resulting data from each sweep can be accurately processed. In addition, multiple sweeps are often required to inject sufficient energy into the ground. Multiple short sweeps can result in better data quality than long sweeps through the use of phase rotations to reduce harmonic noise and by reducing ground roll reverberations. However, the use of multiple sweeps with each sweep followed by a listening time limits the rate at which energy can be put into the ground and the survey acquired. Fourth, the recording of high frequencies can be limited by the simultaneous recording of the signals from an array of vibrators, each vibrator at a different position and elevation and having a different coupling with the ground. Fifth, vibratory sources often produce large amplitude surface coupled waves. These waves, generally referred to as ground roll, are usually in the 10-30 Hz frequency range and have a slower velocity than the desired seismic reflections which can cause the desired reflection data to be obscured by the ground roll. Due to their large amplitude and low frequency nature, the source generated surface waves have the potential to cause structural damage in structures such as buildings, wells and pipelines. There are several methods currently used to mitigate the effects of surface waves: pseudo-random sweeps and high frequency rate sweeps used as the drive signal, frequency dependent force functions (See U.S. Pat. No. 6,152,256), force limiting and distance limiting. The latter two methods are often employed based on PPV (peak particle velocity) measurements taken at a few locations in the survey area. These methods reduce the energy levels that are available to image the subsurface structures. Distance limiting is a particular concern because it can detrimentally alter the offset and azimuth distributions in the seismic data volume. Without mitigating the effects of ground roll, the effectiveness of the subsurface imaging is severely reduced.
An alternative approach for surface wave mitigation is envisioned by the High Fidelity Vibratory Seismic Method (HFVS) disclosed in U.S. Pat. No. 5,719,821 to Sallas, et al. and U.S. Pat. Nos. 5,790,473 and 5,715,213 and 5,550,786 to Allen. In the HFVS method, the recorded seismic data are not correlated with a pilot signal, but instead are inverted using measured vibrator signatures from each sweep and each vibrator. Because the measured signatures include harmonics, the inversion of the corresponding records recovers those harmonics in the processed data, and thereby does not result in additional noise in the data. Because correlation is not used, correlation side lobes do not exist as a potential problem. Furthermore, inversion with a measured vibrator signature can reduce effects from variable vibrator coupling with the earth. However, in this method the vibrator motion for each data record is measured and used in the processing steps. The HFVS method can be used to record multiple source points simultaneously using a number of vibrators. Techniques to date treat sets of vibrators as units but with HFVS, it is possible to treat a single vibrator as a unit which leads to a heretofore unexamined path of single/multiple unit scheduling. The concept of unit scheduling, though more straight-forward with vibratory sources, is equally applicable to impulsive sources.
The separation of a single vibrator's data from the combined data of multiple vibrators allows vibrators to be treated as individual units. Once vibrators are treated as individual units, it is possible to adjust their locations to mitigate surface waves by improved spatial sampling with no additional operating costs.
Data adaptive processes have long been utilized in seismic data acquisition; but have been limited to very local activities such as 1) phase-locking and controlling vibrators and 2) processing the recorded seismic data. U.S. Pat. No. 4,567,583 to Landrum, U.S Pat. No. 3,711,824 to Farron et al. and U.S Pat. No. 4,616,352 to Sallas illustrate the former category and U.S Pat. No. 6,961,283 to Kappius, U.S Pat. No. 6,381,544 to Sallas and U.S Pat. No. 6,651,007 to Ozbek illustrate the latter class of processes that are data adaptive. A data adaptive process may be defined as a process in which one or more control parameters (e.g. a phase shift to lock one signal to another) based on data measurement or measurement statistics. For all of the preceding examples, the source effort and receiver effort are confined to a few predefined parameter sets which are applied to all data acquired. Predominately the recorded data is processed in an adaptive manner without any additional information. Methods described in U.S. Pat. No. 6,381,544 to Sallas and U.S. Pat. No. 3,895,343 to Farr do use ancillary information; but in U.S. Pat. No. 6,381,544 to Sallas, the same parameter set is used for all data and the process adapts to whatever ancillary signal is recorded. In U.S. Pat. No. 3,895,343 to Farr and U.S. Pat. No. 6,381,544 to Sallas, a single source of ancillary information is utilized to alter the source signal and the ancillary information is very local to the energy source. In U.S. Pat. No. 3,895,343 to Farr, the method is not adaptable to impulsive sources. Farr's method does provide a goal oriented solution; but does not provide a method for deciding what to do when goals conflict. Similar arbitration of conflicting goals can be found in unrelated fields such as machine control and process optimization.